[xaml] improve performance in debug-mode

[jonathanpeppers]

Applies to: #18505
Context: https://github.com/dotnet/maui/files/13251041/MauiCollectionView.zip

In reviewing various issues about CollectionView, there is a pattern of:

• My app is slow (in Debug mode)

• Oh, but it seems completely OK in Release!

This is due to several features enabled in Debug mode, such as:

• On mobile, the Mono interpreter is used (enables C# hot reload)

• Xaml compilation is disabled (enables XAML hot reload)

• Other debug features are enabled (such as the $(Optimize) flag)

I attached dotnet-trace to the app above on a Pixel 5 while debugging, just to see what I could find. I also had XAML & C# hot reload enabled.

One thing that appears when parsing XAML is:

  2.11s microsoft.maui.controls.xaml!Microsoft.Maui.Controls.Xaml.ApplyPropertiesVisitor.GetBindableProperty(System.Type,string
257.40ms System.Private.CoreLib!System.String.Format(string,object,object)
172.25ms microsoft.maui.controls!Microsoft.Maui.Controls.Xaml.XamlParseException.FormatMessage(string,System.Xml.IXmlLineInfo)

So about ~2 seconds of this app's startup was in
ApplyPropertiesVisitor.GetBindableProperty(). Looking at what is happening:

if (exception == null && bindableFieldInfo == null)
{
    exception =
        new XamlParseException(
            Format("BindableProperty {0} not found on {1}", localName + "Property", elementType.Name), lineInfo);
}
//...
if (throwOnError)
    throw exception; // only use of `exception`

But then throwOnError is actually false in this case, so a XamlParseException is created and not used. I could just reorder the logic to avoid creating a XamlParseException and calling Format().

Next, I noticed the System.Reflection usage:

var bindableFieldInfo = elementType.GetFields(supportedFlags)
    .FirstOrDefault(fi => (fi.IsAssembly || fi.IsPublic) && fi.Name == localName + "Property");

If this were called on many bindable properties on Label, Button, etc., this code would create lots of FieldInfo[] arrays and iterate until a matching property (field) is found.

I think we can change this to avoid creating FieldInfo[] arrays, such as:

var bindableFieldInfo = elementType.GetField(localName + "Property", supportedFlags);
if (bindableFieldInfo is not null && (bindableFieldInfo.IsAssembly || bindableFieldInfo.IsPublic))
{
    //...

With these changes in place, it significantly improves the call:

816.35ms microsoft.maui.controls.xaml!Microsoft.Maui.Controls.Xaml.ApplyPropertiesVisitor.GetBindableProperty(System.Type,string

Saving over about ~1.3 seconds of startup time in debug mode.

This should improve the performance of XAML parsing on all platforms, which is most likely being used while debugging. You can disable Xaml compilation in Release mode, but it is not recommended.

[StephaneDelcroix]

IIRC, there was a reason using GetFields() instead of GetField(). Could be that one looks up the hierarchy and the other not, or the order, or different versions using different profiles (netstandard1, ...), But it should be caught by unit tests

[StephaneDelcroix]

it even looks like all invocations of GetBindableProperty use false for throwOnError, so this could be simplified, and the signature changed to return a BP? so the intention are clear

[StephaneDelcroix]

there's probably a story behind that, but even I can't remember it

[jonathanpeppers]

I removed the throwOnError parameter. 👍